I have that tool tray in my '87 R80 ( mono ) , also had one on my '81 R80G/S years ago . R 80 required less cranking amps than a R100 , so a smaller battery , and therefore a smaller battery tray was acceptable . The smaller battery tray is mounted transverse and the battery is less than 3" thick . Your R100GS has the battery mounted longitudinally , so the tool tray probably will not work with that ; I felt under mine with tool tray installed and the bottom of tool tray extended below battery tray slightly . Also there is a tab on mine that engages a tab on the fender .